3.2.4. Managing the Size of Clos Topology Tiers

3.2.4. Managing the Size of Clos Topology Tiers

If a data center network size is small, it is possible to reduce the number of switches in Tier 1 or Tier 2 of a Clos topology by a factor of two. To understand how this could be done, take Tier 1 as an example. Every Tier 2 device connects to a single group of Tier 1 devices. If half of the ports on each of the Tier 1 devices are not being used, then it is possible to reduce the number of Tier 1 devices by half and simply map two uplinks from a Tier 2 device to the same Tier 1 device that were previously mapped to different Tier 1 devices. This technique maintains the same bandwidth while reducing the number of elements in Tier 1, thus saving on CAPEX. The tradeoff, in this example, is the reduction of maximum DC size in terms of overall server count by half.

In this example, Tier 2 devices will be using two parallel links to connect to each Tier 1 device. If one of these links fails, the other will pick up all traffic of the failed link, possibly resulting in heavy congestion and quality of service degradation if the path determination procedure does not take bandwidth amount into account, since the number of upstream Tier 1 devices is likely wider than two. To avoid this situation, parallel links can be grouped in link aggregation groups (LAGs), e.g., [IEEE8023AD], with widely available implementation settings that take the whole "bundle" down upon a single link failure. Equivalent techniques that enforce "fate sharing" on the parallel links can be used in place of LAGs to achieve the same effect. As a result of such fate-sharing, traffic from two or more failed links will be rebalanced over the multitude of remaining paths that equals the number of Tier 1 devices. This example is using two links for simplicity, having more links in a bundle will have less impact on capacity upon a member-link failure.